The present invention relates to a shadow mask type cathode ray tube, which is used for a television receiver, a computer display, and the like.
FIG. 4 is a cross-sectional view showing one example of a conventional color cathode ray tube. The color cathode ray tube 1 shown in FIG. 4 includes a substantially rectangular-shaped face panel 2 having a phosphor screen 2a on its inner face, a funnel 3 connected to the rear side of the face panel 2, an electron gun 4 contained in a neck portion 3a of the funnel 3, a shadow mask 6 facing the phosphor screen 2a inside the face panel 2, and a mask frame 7 for fixing the shadow mask 6. Furthermore, in order to deflect and scan electron beams, a deflection yoke 5 is provided on the outer periphery of the funnel 3.
The shadow mask 6 plays a role of selecting colors with respect to three electron beams emitted from the electron gun 4. xe2x80x9cAxe2x80x9d shows a track of the electron beams. The shadow mask is provided with a number of apertures formed by etching, through which electron beams pass.
In a color cathode ray tube, due to the thermal expansion caused by the impact of the emitted electron beams, the electron beam through aperture is shifted. Consequently, a doming phenomenon occurs. That is, the electron beams passing through the apertures fail to hit a predetermined phosphor correctly, thus causing unevenness in colors. Therefore, a tension force to absorb the thermal expansion due to the temperature increase of the shadow mask is applied in advance, and then the shadow mask is stretched and held to the mask frame. When the shadow mask is stretched and held as mentioned above, even if the temperature of the shadow mask is raised, it is possible to reduce the amount of displacement between an aperture of the shadow mask and phosphor strips of the phosphor screen.
FIG. 5 is a plan view showing an example of a shadow mask 35 to which a tension force is applied mainly in the vertical direction (vertical direction of the screen). Apertures 36 are formed at constant pitches. Reference numeral 37 is referred to as a bridge, which is a portion between respective apertures 36. The bridge width has an effect on the mechanical strength of the shadow mask. More specifically, the bridge with a narrow width has a weak tension force particularly in the horizontal direction. If the bridge width is increased in order to improve the mechanical strength, the open area of the aperture is reduced, thus deteriorating the luminance intensity.
Furthermore, the vertical pitch of the bridge is related to the doming amount of the shadow mask. The shadow mask is stretched mainly in the vertical direction. Therefore, the thermal expansion in the vertical direction is absorbed by the tension force, while the thermal expansion in the horizontal direction is transmitted in the horizontal direction through the bridges. The doming amount can be reduced by increasing the vertical pitch of the bridge. Therefore, when the vertical pitch of the bridge is increased, the doming amount can be reduced. In this case, however, moire stripes easily occur, thus causing the deterioration of the image quality. The moire stripe means a mutual interference stripe between scanning lines (luminescent lines) of the electron beams arranged at constant intervals and the regular pattern of the electron beam through apertures of the shadow mask.
Furthermore, when the vertical pitch of the bridge is increased, the bridges themselves may appear as dots on the screen, or may be recognized as a pattern in which the bridges are piled up (a brick-like pattern).
On the contrary, when the vertical pitch of the bridge is reduced, moire stripes are suppressed sufficiently and the bridges themselves are not noticeable, but the doming amount is increased.
To solve this problem, as shown in FIG. 6A, for example, a shadow mask 40 is proposed in which protruding portions 42a, 42b protruding in different directions are formed in an aperture 41. By forming the protruding portions 42a, 42b, the vertical pitch of the bridge is maintained at a large value, while the occurrence of moire stripes can be suppressed in the same manner as the vertical pitch of the bridge is reduced. In other words, it is possible to reduce the doming amount of the shadow mask to which a tension force is applied mainly in the vertical direction and also to suppress the occurrence of moire stripes at the same time.
Furthermore, a shadow mask provided with slot-shaped apertures shows a so-called xe2x80x9cpersimmon stone phenomenonxe2x80x9d, which is characterized as follows. By taking the vertical center line of the shadow mask as the border, electron beams entering obliquely into apertures located on the right side of the phosphor screen 2a (FIG. 4) are cut in the vicinity of upper right and lower right corners of the apertures, and electron beams entering obliquely into apertures located on the left side are cut in the vicinity of upper left and lower left corners of the apertures.
FIG. 7A is a horizontal cross-sectional view showing the vicinity of upper and lower corners of an aperture located on the right side of the phosphor screen by taking the vertical center line of the shadow mask as the border. FIG. 7A shows a state in which a portion of an electron beam ray 51 passing through an aperture 50 is cut by an ascending portion 52 of the aperture 50. In this way, when the electron beam is cut in the vicinity of the upper right and lower right corners of the aperture, as illustrated in FIG. 7B, the shape of a beam spot 53 that originally is intended to be of a substantially slot shape is formed into the shape of a persimmon stone. Shaded portions 54a and 54b are portions where the electron beam ray was cut.
In order to prevent this persimmon stone phenomenon in the vicinity of the upper and lower corners of the apertures, various shapes of apertures are proposed. For example, JP1(1989)-320738A discloses a method for preventing this persimmon stone phenomenon by increasing a width of an open portion (i.e. W in FIG. 7A) in the vicinity of upper right and lower right corners of apertures located on the right side of a shadow mask from the vertical center line. Furthermore, JP63(1988)-119139A discloses another method for preventing electron beams from being cut, as shown in FIG. 7C, by widening upper and lower end portions a, b of an aperture 55.
Although the shadow mask 40 shown in FIG. 6A can suppress the occurrence of moire stripes with the protruding portions 42a, 42b by shielding the electron beam in the same manner as the vertical pitch of the bridge is reduced, this shadow mask still suffered from this persimmon stone phenomenon. In other words, as illustrated in FIG. 6B, the electron beam is cut in base portions C with regard to the aperture 41 located on the right side of the shadow mask from the vertical center line, and the electron beam is cut in base portions D with regard to the aperture 41 located on the left side from the vertical center line of the shadow mask.
Furthermore, the apertures proposed by JP1(1989)-320738A and JP63(1988)-119139A both try to prevent this persimmon stone phenomenon by changing the aperture shape, but there is a limitation to solve the above problem with the shape.
It is an object of the present invention to solve the conventional problems described above by providing a cathode ray tube capable of reducing the doming amount and suppressing the occurrence of moire stripes at the same time, and which can prevent the so-called persimmon stone phenomenon from occurring.
To achieve the above object, a cathode ray tube of the present invention includes a shadow mask made of a flat plate provided with a number of apertures, wherein protruding portions protruding in the horizontal direction from an end of the aperture that is closer to the vertical center line of the shadow mask are formed at least in the apertures positioned in the vicinity of both right and left edges of the shadow mask. According to the configuration described above, while the cathode ray tube can reduce the doming amount and suppress the occurrence of moire stripes at the same time, the persimmon stone phenomenon in which the beam spot of an electron beam on the phosphor surface is cut partially can be prevented from occurring.
In the cathode ray tube described above, it is preferable that an end of the aperture that is farther from the vertical center line has dented portions in areas opposing tips of the protruding portions. According to this configuration, the dented portions are formed so that the protruding portions can be lengthened and the area of the aperture can be enlarged at the same time. Thus, the occurrence of moire stripes and the persimmon stone phenomenon can be suppressed even more surely.
Furthermore, it is preferable that the protruding portions are formed almost on the entire shadow mask.
Furthermore, it is preferable that the shadow mask is stretched and held in a state in which a tension force is applied in the vertical direction.
Furthermore, it is preferable that the shadow mask is formed into a curved shape.